1. Field of the Invention
This invention is in the field of very large optical apertures, and in particular to the use of thin membranes with curvature as mirror substrates.
2. Description of the Prior Art
Various focusing mirror systems fabricated from a reflective metalized membrane are known in the prior art. Commonly, a differential pressure is established between an enclosed area behind the reflective surface and the ambient pressure to control the contour of the flexible reflective surface. The curvature is controlled by various means, such as: an electropneumatic control system (U.S. Pat. No. 4,179,193); an actuator pushing or pulling on a rear membrane (U.S. Pat. No. 5,016,998): an actuator in physical contact with the rear surface of the membrane (U.S. Pat. No. 4,422,723); a double membrane with a partial vacuum between with a complex edge tensioning system to vary the curvature (U.S. Pat. Nos. 5,680,262 and 5,552,006); and a curvature determined by uniform differential pressure applied to a membrane with a non-uniform radial distribution of thickness or a uniform membrane loaded with a non-uniform differential pressure obtained by localized electrostatic or magnetic pressure (U.S. Pat. No. 4,046,462).
Most of the aforementioned inventions are designed for solar energy concentrators. The curvatures obtained do not approach the optical quality required of an astronomical telescope. Optical quality telescopes used with real time monochromatic holography, for example, should have surfaces that deviate no more than 40 lines per mm of localized tilt and 200 waves of a low spatial frequency aberration for any given mirror figure. This situation should produce a near (1.6.times.) diffraction limited image. Very large optical apertures, particularly for space-based systems, could benefit from lightweight, optical quality membrane mirrors. Potential applications include astronomy, imaging and surveillance, and laser beam projection.